The invention relates to an amplifier for supplying a signal to a load, comprising a first transistor having a first main terminal coupled to a reference terminal, a control terminal, and a second main terminal for supplying the signal to the load, and sensing means for determining the value of the signal.
Such an amplifier is known from the U.S. Pat. No. 5,285,143.
In the known amplifier, a sense current is generated from a load current which is scaled down such that the sense current is an accurate representation of the load current but has a substantially smaller magnitude. The scaled down sense current is generated by coupling a power sense resistor in series with the load current and develops a load voltage across this resistor. A sense voltage, being substantially equal to the load voltage, is applied to a pilot sense resistor by a voltage follower. The pilot sense resistor is some predetermined ratio of the power sense resistor. The sense current flowing through the pilot sense resistor is scaled down in accordance with the ratio of the power resistor to the pilot sense resistor. A current mirror supplies current to the voltage follower.
A problem of the known amplifier is that a sensing element, for example formed by a resistor, is coupled in series with the first transistor. Though a high accuracy of the sense signal (current) can be accomplished in this manner in principle, the circuitry needed in order to reach this accuracy is quite complicated.
It is an object of the invention to provide an amplifier which eliminates the above-mentioned problem.
To this end, according to the invention, the amplifier of the type defined in the opening paragraph is characterized in that the sensing means comprises a second transistor having a first main terminal coupled to the first main terminal of the first transistor, a control terminal coupled to the control terminal of the first transistor, and a second main terminal for supplying a further signal which is a representation of the signal, and in that the amplifier comprises detection means for supplying a DC-component of the further signal.
The invention is based on the insight that for many applications, and especially for mobile phones, amplifiers do not operate in the so called class A mode in which the power consumption is relatively high, but operate in a lower-energy mode, for example in the so called class B or class C mode. In these modes the signal comprises a DC component which is proportional to the signal and is thus also proportional to the power supplied to the load. Since the further signal is a representation of the signal supplied to the load, the further signal also comprises a DC-component which is a measure for the power supplied to the load. This measure can be used to monitor the amount of power supplied to the load and possibly to regulate the power to a desired level.
An embodiment of the invention may be characterized in that the detection means comprises a resistor coupled between a power supply terminal and the second main terminal of the second transistor, and a capacitor coupled between the second main terminal of the second transistor and the reference terminal. By so doing, a simple implementation of the detection means is accomplished. As an alternative solution the capacitor may also be connected between the second main terminal of the second transistor and the power supply terminal.